Contact with nature may benefit, not only the bodily organism, but also the psychological self. We proposed that, assuming humans' innate affinity for nature (the biophilia hypothesis), nature would be conducive to a sense of environment-self fit, which would be experienced as authenticity (being aligned with one's true self). We formulated several hypotheses: (a) nature fosters authenticity, and it does so through at least four plausible mechanisms: self-esteem, basic needs satisfaction (autonomy, competence, relatedness), mindfulness, and positive affect; (b) self-esteem is the strongest mechanism overall, and autonomy is the strongest mechanism of the three basic needs; (c) self-esteem and authenticity mediate sequentially the positive impact of nature on current psychological well-being (higher life satisfaction and meaning in life); and (d) authenticity mediates the positive influence of nature on longer term psychological well-being (higher life satisfaction and meaning in life, lower depression, anxiety, and stress). We obtained support for these hypotheses across 12 studies (N = 5,512). These were diverse in terms of setting (field, laboratory), design (cross-sectional, experimental, longitudinal), methodology (varying manipulations of nature and assessment of mediators and/or dependent measures), and sampling (university/community, East Asian/Western). The findings establish nature as a correlate and determinant of authenticity, chiefly via the mechanism of self-esteem, and further establish authenticity (preceded by self-esteem) as a mediator of the positive influence of nature on psychological well-being. The findings are also generative and have policy implications.